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Published June 16, 2003
Research Roundup
Cells expressing ERas form tumors in mice.
connected to the LIF/Stat3 pathway, which is dispensable for normal mouse development but necessary in ES cells and for the extended survival of mouse blastocysts (a process called diapause). Only in the ES cells and during diapause must pluripotency be extended past a few days, and this extension may be helped by LIF or ERas expression. The situation is clouded further in humans and monkeys, however, as both have apparently functional ERas genes but lack diapause. Whatever the function of ERas, “ERas null cells should be much safer for clinical applications,” says Yamanaka. ERas has residues characteristic of activated Ras proteins, and is found largely in the
activated, GTP-bound form. Its addition transforms cells, whereas its deletion from ES cells sharply reduces their tumor-forming ability. Growth is also reduced in ERas knockout cells, but Yamanaka says that rich culture conditions largely correct for this and thus slow growth should not hold back the use of altered ES cells. Direct human applications will not, however, come from Yamanaka’s group in the foreseeable future, as there are extensive bureaucratic hurdles involved in experimentation on human ES cells in Japan. Reference: Takahashi, K., et al. 2003. Nature. 423:541–545.
WASPs with memory
M
emory may not be confined to brain cells. Eduardo Torres and Michael Rosen (University of Texas Southwestern Medical Center, Dallas, TX) have found that the actin polymerization activator WASP has at least the potential to be a memory device. Such a capacity could switch cells into an altered state in which they respond more acutely to a stimulus the second time around. Rosen was intrigued initially by the location of a tyrosine in WASP that others had found was phosphorylated. In structural models, Tyr 291 is buried in the fold that forms in the autoinhibisted form of WASP. Sure enough, Tyr 291 could only be phosphorylated (thus further activating WASP activity) when the structure was opened up via addition of the activating Cdc42. Both Tyr 291 phosphorylation and Cdc42 activation can be driven by Src family kinases. But Cdc42 In inactive WASP, activation and opening of WASP Tyr 291 (purple) take some time, so only a is hidden between two domains. persistent Src signal will still Rosen/Elsevier
The Journal of Cell Biology
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1008 The Journal of Cell Biology | Volume 161, Number 6, 2003
be around to phosphorylate Tyr 291 by the time it is exposed. WASP could then act as a memory device if Src maintains the Tyr 291 phosphorylation until after Cdc42 is turned off (as might happen if a GTPase activating protein drops by and shuts off Cdc42). Torres and Rosen showed that this form of WASP clasps and protects the phosphorylation on Tyr 291 so it is resistant to removal by phosphatases. In contrast to the initial, unphosphorylated form of WASP that took such a sustained signal to activate, the phosphorylated WASP is now in a primed form. A future signal need only tickle the Cdc42 system to achieve full activation. For now, the site of action of these mechanisms are unknown. “What we’ve found is exclusively biochemical,” says Rosen. “We don’t know where this will occur in vivo.” But, he says, “having this as the biochemical concept will drive the kind of work” needed to put the findings in context. Rosen is looking for places where later responses might be affected by earlier experience. The best candidate is in neurite outgrowth, where N-WASP phosphorylation has been seen to persist long after the kinase stimulus has vanished. Perhaps such phenomena allow neurites to keep extending even through areas that have temporary drops in outgrowth factors. Reference: Torres, E., and M.K. Rosen. 2003. Mol. Cell. 11:1215–1227.
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n the debate over whether ES cells or somatic stem cells have better prospects for stem cell therapies, ES cell proponents admit that their pluripotent cells carry an increased risk: ES cells can form teratomas. But now Kazutoshi Takahashi, Kaoru Mitsui, and Shinya Yamanaka (Nara Institute of Science and Technology, Nara, Japan) have found that a newly discovered Ras gene may be at the heart of this problem. ERas, previously thought to be a pseudogene, was found by the group as being expressed specifically in ES cells. The other genes in this expression class are also expressed in early embryonic tissues such as the inner cell mass and epiblast, but no expression of ERas is evident in the embryo. The lack of phenotype of ERas knockout mice adds to the mystery. Yamanaka suggests that ERas may be
Yamanaka/Macmillan
A tumorigenic culprit in ES cells
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Published June 16, 2003
TEXT BY WILLIAM A. WELLS
[email protected]
Helicobacter breaks down junctions
Amieva/AAAS
here is a time for caution and a time for and that the new Ser 214 phosphorylation committing. For the cell cycle, a halt blocks reestablishment of the earlier Ser 216 before mitosis is an appropriate response to phosphorylation. Thus, Cdc25C is locked ionizing radiation or inhibition of DNA on in mitotic cells, and does not respond replication. But once mitosis to irradiation. is underway the cell is better The mitosis-reinforcing off just carrying on regardless function of the Ser 214 is clear of DNA-damaging insults. from a Ser 214 to Ala mutant, Dmitry Bulavin, Albert Forwhich delays entry into mitosis nace (National Cancer Institute Ser 214 phosphorylation in mammalian cells and [NCI], Bethesda, MD), and reinstates a DNA damage (red) keeps Cdc25C colleagues now report on a replication checkpoint that switched on only in double phosphorylation switch mitotic cells (green). is normally absent in early embryonic frog extracts. The Ser 214 residue that keeps irradiated mitotic cells from is probably phosphorylated by Cdc2 itself, bouncing unexpectedly out of mitosis. but it is unclear how the inactive Cdc25C The switch is in one of the Cdc25 and Cdc2 might jumpstart each other— phosphatases—proteins that remove an other Cdc25 isoforms or polo kinase are inhibitory phosphate from Cdc2, thus allowing entry into mitosis. Cdc25C is itself possibilities. Meanwhile, the authors suspect phosphorylated during interphase. First, a that pairs of mutually exclusive phosphoryconstitutive kinase and then an irradiation- lation sites will turn up in other regulatory induced kinase hit Ser 216 on Cdc25, thus proteins, especially those that, like Cdc25C, keeping the phosphatase inactive. The NCI bind to 14–3-3 proteins. team now show that this Ser 216 phosReferences: Bulavin, D.V., et al. 2003. Nat. phorylation is replaced in mitotic cells by Cell Biol. 5:545–551. phosphorylation of Cdc25C on Ser 214, Bulavin, D.V., et al. 2003. Cell Cycle. 2:263–266.
T
Fornace/Macmillan
The Journal of Cell Biology
Committing to mitosis
pair of proteins, each individually incapable of maintaining a polar distribution, can convince each other to stay put at the ends of a fission yeast cell, according to Hilary Snaith and Ken Sawin (University of Edinburgh, Edinburgh, Scotland).
A
Tea1p (green) travels along microtubules but can only be anchored in the presence of mod5p (left).
Polarity studies in fission yeast have focused on the tea1p protein. It can be seen hitching a ride on growing microtubules as they speed toward the two ends of the cell—the only sites where growth takes place in fission yeast. Now, Snaith and Sawin have found a protein called mod5p that is localized to cell ends and helps to keep the arriving tea1p anchored to those same sites. Cells lacking mod5p delivered tea1p as usual but failed to keep it localized at the cell ends. Mod5p, in turn, was found all around the plasma membrane when tea1p was no longer present. Thus, the authors believe that tea1p and mod5p feedback on each other to ensure immobility. There is preliminary evidence to suggest an indirect physical link between tea1p and mod5p. This linkage may cement both proteins in a complex that is big enough or sticky enough to be inherently immobile, thus anchoring them near the site where tea1p arrives. Alternatively, the association of the two proteins may trigger a biochemical change in the complex that fixes the complex in place. Either way, Sawin believes that the next step will be more biochemistry to determine just what happens when tea1p is dropped off at the ends. Reference: Snaith, H.A., and K.E. Sawin. 2003. Nature. 423:647–651.
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Reference: Amieva, M.R., et al. 2003. Science. 300:1430–1434.
Anchored by feedback
Sawin/Macmillan
B
acteria can break apart cell junctions that link neighboring stomach cells, based on the work of Manuel Amieva, Roger Vogelmann, Stanley Falkow (Stanford University, Stanford, CA), and colleagues. Although the bacteria may do it to gain access to tasty chemicals that leak out, the results for humans may include stomach ulcers and gastric cancer. The link from cell junctions to stomach ailments may, say the researchers, lie in tissue repair. Injury to the stomach triggers cell division and migration to plug the gap. The chiefs in charge of these processes may well lie in cell junctions—ideally placed, as they would be, to sense whether there is a breach in the epithelium. If bacterial proteins interfere with that process, the persistent gaps could lead to ulcers. And if the bacterial proteins push the repair process into inappropriate overdrive then cancerous growths might arise. Such pathways remain the stuff of speculation. But what the H. pylori (Hp) opens Stanford team has shown is that CagA, a protein that the ulcerup cell junctions associated bacterium Helicobacter pylori injects into gastric to a black dye epithelial cells, can associate and interfere with junctional proteins. (arrowhead). Some of the tight junction scaffolding protein ZO-1 is lured away from junctions to associate with attached bacteria, and still more ZO-1 colocalizes with intracellular CagA at the remaining tight junctions, which are now leaky. Others have demonstrated that CagA can bind signaling proteins such as SHP2 and Grb2 and increase spreading of isolated cells driven by the c-Met receptor. Although these effects are initially resisted in monolayers, the cells eventually succumb, perhaps when CagA induces inappropriate signaling from what is left of the cell junctions.
